High strength aluminum alloy fin stock for heat exchanger

ABSTRACT

The present invention provides an aluminum alloy fm stock alloy material with higher strength, and improved sag resistance for use in heat exchangers. This aluminum alloy fm stock alloy material was made by direct chill (DC) casting.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. provisional patentapplication Ser. No. 61/863,568 filed Aug. 8, 2013, which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the fields of material science,material chemistry, metallurgy, aluminum alloys, aluminum fabrication,and related fields. The present invention provides novel aluminum alloysfor use in the production of heat exchanger fins, which are, in turn,employed in various heat exchanger devices, for example, motor vehicleradiators, condensers, evaporators and related devices.

BACKGROUND

There is a need for aluminum alloy fin stock with high strength andimproved sag resistance high strength for use in various heat exchangerapplications including radiators for automobiles. There is a need toobtain aluminum alloy fin stock with strong pre-braze mechanicalproperties, good behavior during brazing, i.e., enhanced brazed materialsag resistance and reduced fin erosion, and good strength andconductivity characteristics after braze for high performance heatexchanger applications.

SUMMARY

The present invention provides an aluminum alloy fin stock alloymaterial with higher strength, and improved sag resistance for use inheat exchangers. This aluminum alloy fin stock alloy material was madeby direct chill (DC) casting.

A DC fin stock material was developed with desirable pre-braze (H14temper) and post-braze mechanical properties, sag resistance, corrosionresistance and conductivity. The aluminum alloy fin stock alloy displayslarger grain and improved strength before brazing.

The aluminum alloy fin stock alloy can be used in various applications,for example heat exchangers. The finstock is particularly useful forhigh performance light weight, automotive heat exchangers but could beused for other brazed applications including but not limited to HVAC. Inone embodiment, the aluminum alloy fin stock alloy can be used inautomotive heat exchangers such as radiators, condensers andevaporators. Other objects and advantages of the invention will beapparent from the following detailed description of embodiments of theinvention.

DESCRIPTION

The present invention provides an aluminum alloy fin stock alloymaterial with higher strength, improved corrosion resistance andimproved sag resistance for use in heat exchangers, such as automotiveheat exchangers. This aluminum alloy fin stock alloy material was madeby direct chill casting.

This DC fin stock material exhibits desirable pre-braze (H14 temper) andpost-braze mechanical properties, sag resistance, corrosion resistanceand conductivity. The aluminum alloy fin stock alloy displays largergrain and improved strength before brazing.

The aluminum alloy fin stock alloy can be used in various applications,for example heat exchangers. In one embodiment, the aluminum alloy finstock alloy can be used in automotive heat exchangers such as radiators,condensers and evaporators.

In one embodiment, the DC fin stock material comprises about 0.8-1.4%Si, 0.4-0.8% Fe, 0.05-0.4% Cu, 1.2-1.7% Mn and 1.2-2.3% Zn, remainderaluminum. All % values are in weight (wt) %.

In another embodiment, the DC fin stock material comprises about0.9-1.3% Si, 0.45-0.75% Fe, 0.10-0.30% Cu, 1.3-1.7% Mn and 1.30-2.2% Zn,remainder aluminum.

In yet another embodiment, the DC fin stock material comprises about0.9-1.2% Si, 0.50-0.75% Fe, 0.15-0.30% Cu, 1.4-1.6% Mn and 1.4-2.1% Zn,remainder aluminum.

Optionally, Cr and/or Zr or other grain size controlling elements may bepresent in these alloy compositions up to 0.2% each, up to 0.15% %, upto 0.1%, up to 0.05%, or up to 0.03%. All % values are in weight (wt) %.

It is to be understood that the alloy compositions described herein maycontain other minor elements sometimes referred to as unintentionalelements, below 0.05%.

Method of Making the Ingots

The ingots described herein are made with a Direct Chill (DC) process,which is commonly used throughout the aluminum sheet industry, whereby alarge ingot ˜1.5 m×0.6 m×4 m is cast from a large holding furnace whichsupplies metal to a shallow mold or molds supplied with cooling water.The solidifying ingot is continuously cooled by the direct impingementof the cooling water and is withdrawn slowly from the base of the molduntil the full ingot or ingots are completed. Once cooled from thecasting process, the ingot rolling surfaces are machined to removesurface segregation and irregularities. The machined ingot is preheatedfor hot rolling. The preheating temperature and duration are controlledto low levels to preserve a large grain size and high strength after thefinished fin stock is brazed. The ingot is hot rolled to form a coilwhich is then cold rolled. The cold rolling process takes place inseveral steps and an interanneal in the range of about 300-450° C. isapplied to recrystallize the material prior to the final cold rollingstep. Next the material is cold rolled to obtain the desired final gaugeand slit in narrow strips suitable for the manufacture of radiators andother automotive heat exchangers. A pre-heat of the ingots prior to hotrolling is conducted in such a way that the final metal temperatureachieved is about 480° C. and is held there for an average of about 4hours (typically a minimum of about 2 hours and a maximum of about 12hours). Several ingots (about 8 to 30) are charged to a furnace andpreheated with gas or electricity to the rolling temperature. Aluminumalloys are typically rolled in the range of about 450° C. to about 560°C. If the temperature is too cold, the roll loads are too high, and ifthe temperature is too hot, the metal may be too soft and break up inthe mill. In this case the preheat temperature is low relative to otheraluminum products and the hold time is relatively short, to limit thegrowth of dispersoids that would decrease the final post braze grainsize. In practice a hot mill is scheduled to roll many different ingotsand alloys and cannot always roll the ingots at minimum soak time. Inone embodiment, the minimum soak time at about 480° C. is about 2 hours.During production, the inter-anneal temperature applied was about 400°C. for an average of about 3 hours followed by applying % cold work (CW)of about 29% to final gauge. The % CW is the degree of cold rollingapplied to get the material in the final required strength range. The %cold work is defined as: (initial gauge−final gauge)*100/initial gauge.As cold work increases, the H14 strength increases, but final post brazegrain size and sag resistance is decreased. 29% is relatively low formost aluminum rolling applications.

In one embodiment a pre heat practice at about 480° C. for an average of4 hours is employed with an interanneal temperature of about 300-400° C.and % CW of about 25-35% to final gauge.

The finished cold rolled coil is then slit into many narrow strips ofthe width required by the heat exchanger manufacturer for forming,assembly and brazing into the finished heat exchanger.

The following example will serve to further illustrate the presentinvention without, at the same time, however, constituting anylimitation thereof. On the contrary, it is to be clearly understood thatresort may be had to various embodiments, modifications and equivalentsthereof which, after reading the description herein, may suggestthemselves to those skilled in the art without departing from the spiritof the invention.

EXAMPLE

A DC case alloy composition was made. The composition range of the alloywas within the following specification: 1.1±0.1% Si, 0.6±0.1% Fe,0.2±0.05% Cu, 1.4±0.1% Mn and 1.50±0.1% Zn with the remainder aluminum.The alloy material had a minimum ultimate tensile strength of ˜130 MPa.The alloy material had an average conductivity after brazing of ˜43 IACS(International Annealed Copper Standard (i.e., pure copper is 100%conductivity)) and an open circuit potential corrosion value (vs.Standard Calomel Electrode (SCE)) of −741 mV. The alloy materialproduced exhibited a sag value between 28 mm where the final gauge was49 μm, and 43 mm where the final gauge was 83 μm, which was within therequired specifications at these gauges. These values were measuredafter applying a simulated brazing cycle whereby the sample was heatedto a temperature of 605° C. and cooled to room temperature in a periodof about 20 minutes to simulate the temperature time profile of acommercial brazing process. The alloy material produced varied in gaugebetween 49 and 83 μm.

All patents, patent applications, publications, and abstracts citedabove are incorporated herein by reference in their entirety. Variousembodiments of the invention have been described in fulfillment of thevarious objectives of the invention. It should be recognized that theseembodiments are merely illustrative of the principles of the presentinvention. Numerous modifications and adaptations thereof will bereadily apparent to those of skill in the art without departing from thespirit and scope of the invention as defined in the following claims.

1. An aluminum alloy comprising about 0.8-1.4 wt % Si, 0.4-0.8 wt % Fe,0.05-0.4 wt % Cu, 1.2-1.7 wt % Mn and 1.20-2.3 wt % Zn with theremainder as Al.
 2. The aluminum alloy of claim 1, comprising about0.9-1.3 wt % Si, 0.45-0.75 wt % Fe, 0.10-0.3 wt % Cu, 1.3-1.7 wt % Mnand 1.30-2.2 wt % Zn, with the remainder as Al.
 3. The aluminum alloy ofclaim 1, comprising about 0.9-1.2 wt % Si, 0.5-0.75 wt % Fe, 0.15-0.3 wt% Cu, 1.4-1.6 wt % Mn and 1.4-2.1 wt % Zn, with the remainder as Al. 4.The aluminum alloy of claim 1, further comprising up to 0.2 wt % of oneor both of Cr or Zr.
 5. An aluminum alloy fin stock material producedfrom the aluminum alloy of claim 1 by a process, comprising: directchill casting the aluminum alloy into an ingot; preheating the ingot to450 to 560° C. for 2 to 16 hours; hot rolling the preheated ingot; coldrolling the ingot; inter-annealing at a temperature of 300-450° C.; and,after inter-annealing, performing a final cold rolling step to achieve %cold work (%CW) of 25-35%.
 6. The aluminum alloy fin stock material ofclaim 5, wherein the ingot is preheated at 480° C. for 2-12 hours. 7.The aluminum alloy fin stock material of claim 5, wherein theinter-annealing temperature is 300-400° C.
 8. The aluminum alloy finstock material of claim 5, having a minimum ultimate tensile strength of˜130 MPa, measured after brazing.
 9. The aluminum alloy fin stockmaterial of claim 5, having a corrosion potential of −700 mV or less,measured after brazing.
 10. A heat exchanger comprising the aluminumalloy of claim 1 or the aluminum alloy fin stock material of claim 5.11. The heat exchanger of claim 10, wherein the heat exchanger is anautomotive heat exchanger.
 12. The heat exchanger of claim 10, whereinthe heat exchanger is a radiator, a condenser or an evaporator.
 13. Useof the aluminum alloy of claim 1 or the aluminum alloy fin stockmaterial of claim 5 for fabrication of heat exchanger fins.
 14. Aprocess for making an aluminum alloy fin stock material, comprising:direct chill casting the aluminum alloy of claim 1 into an ingot;preheating the ingot to 450 to 560° C. for 2 to 16 hours; hot rollingthe preheated ingot; cold rolling the ingot; inter-annealing at atemperature of 300-450° C.; and, after inter-annealing, performing afinal cold rolling step to achieve % cold work (%CW) of 25-35%.
 15. Theprocess of claim 14, wherein the ingot is preheated at 480° C. for 2-12hours.
 16. The process of claim 14, wherein the inter-annealingtemperature is 300-400° C.
 17. The process of claim 15, wherein theinter-annealing temperature is 300-400° C.